As flow sensors disposed in electronically controlled fuel injectors of internal combustion engines for, e.g., automobiles to measure an amount of intake air, thermal flow sensors have been prevalently employed in the past because of the capability of directly detecting an air amount by mass.
Of those thermal flow sensors, attention has been focused particularly on a flow sensor fabricated using semiconductor micromachining techniques for the reason that such a thermal flow sensor can be produced at a lower cost and can be driven with smaller power. That type of known thermal flow sensor using a semiconductor substrate is disclosed, for example, in JP-A-2001-41790 filed by the applicant of this application.
The known thermal flow sensor disclosed in JP-A-2001-41790 is of the so-called temperature difference type in which, over a cavity formed in a semiconductor substrate, a heating resistance is formed with an electrical insulation film interposed between the heating resistance and the cavity, a pair of temperature measuring resistances are arranged upstream and downstream of the heating resistance in spaced relation, and a flow rate is measured from a temperature difference between the upstream and downstream temperature measuring resistances.
That thermal flow sensor of the temperature difference type measures the flow rate from a change of temperature distribution (temperature difference) in the electrical insulation film lying over the cavity in the direction of airflow, but the temperature distribution (measurement accuracy) is greatly affected by respective shapes of the electrical insulation film, the heating resistance, and the temperature measuring resistances arranged upstream and downstream of the heating resistance, which are all positioned over the cavity, as well as by relative positional relationships among those members.
In the above-mentioned related art, the respective shapes of the electrical insulation film, the heating resistance, and the temperature measuring resistances arranged upstream and downstream of the heating resistance, which are all positioned over the cavity, as well as the relative positional relationships among those members are specifically defined for the purposes of improving symmetry of the temperature distribution in the electrical insulation film lying over the cavity, widening a range of measurable flow rate, and realizing a high-speed response.
However, the disclosed arrangement has the problem as follows. Because of not sufficiently taking into consideration reliability resulting when the thermal flow sensor is mounted in an internal combustion engine for an automobile, etc. and is used under severe environmental conditions for a long term, there occurs a phenomenon that floating fine particles, such as carbon particles, are deposited thick on the electrical insulation film lying over the cavity due to the thermophoretic effect during use for a long term. Accordingly, the accuracy in measurement cannot be maintained at a sufficient level for a long term.
Further, the related art trying to cope with the above-described contamination is disclosed, for example, in JP-A-11-230802 that was also filed by the applicant of this application.
According to the related art disclosed in JP-A-11-230802, a photo-catalyst film is formed on a protective film covering a heating resistance, and a light emitting means is provided to illuminate light to the photo-catalyst film, thereby reducing an influence of the contamination based on the photo-catalyst effect.